Switching power converters offer higher efficiency as compared to linear regulators. Although linear regulators are relatively inexpensive, they regulate a lower output voltage from a higher input voltage by simply burning the difference as heat. As a result, a linear regulator typically burns more power than is actually supplied to the load. In contrast, a switching power converter regulates its output voltage by delivering relatively small increments of energy through the cycling of a power switch. The power switch in a switch-mode device is either off or on such that efficiency is markedly improved as compared to linear regulators.
Given their high efficiency, a switching power converter such as a flyback converter are typically used as the power supply for solid state lighting applications. A flyback converter includes a transformer having a primary winding and a secondary winding. A rectified AC voltage powers the primary winding when a primary-side controller cycles the power switch on. The resulting current through the primary winding develops a magnetic field that stores energy. This stored energy is released when the power switch cycles off, resulting in a pulse of secondary current. The secondary current powers the solid state lighting bulb (a light emitting diode (LED)).
Although flyback converters have high efficiency, their integration in solid state lighting applications may be complicated if the user retains traditional triac-based dimming switches. As the user adjusts the dimming level to full output, the triac within the dimming switch passes the full AC cycle from the AC mains to the rectifier that produces the rectified AC voltage for driving the primary winding. But at reduced dimming levels, the triac switch cuts out part of the AC cycle depending upon whether the triac switch is in a leading edge or trailing edge dimming configuration. The greater efficiency of LED bulbs as compared to incandescent bulbs often results in the solid state lighting application not drawing enough current through the triac switch at higher dimming setting such that the triac resets. The LED bulb may then flicker.
To eliminate flicker, triac switches are being phased out in modern solid-state lighting applications. For example, a user may control a voltage or current on the secondary side of the flyback converter through a non-triac-based dimming switch. The dimming setting must then be communicated to the primary-side controller of the power switch so that the appropriate power setting (e.g., a pulse-width-modulation or pulse-frequency-modulation setting) for the power switch may be applied to effect the desired dimming level. But a direct electrical signal such as through coupling a wire or lead from the secondary side to the primary side for communicating the desired dimming level or setting destroys the desired isolation between the primary and secondary windings. To maintain this isolation, the dimming command may be communicated through an isolating device such as an optocoupler or an isolating transformer. For example, the dimming switch may drive a pulse through the optocoupler such as used in conventional binary signaling of a logical one or zero bit value. But optocouplers may have pronounced non-linearity with regard to their rise and fall times due to their current transfer ratios that vary from device to device and also with temperature. As shown in FIG. 1, an input pulse train 100 transmitted through an optocoupler will thus result in a delayed output pulse train 105 having slow rise and fall times. A receiver that compares output pulse train 105 to a threshold voltage 110 produces a received pulse train 115 that is markedly distorted compared to input pulse train 100. In order to reliably produce binary one and binary zero values in the received pulse train 115, the data rate for the input pulse train 100 must be sufficiently slow. If the data rate is increased, a pulse width for input pulse train 100 may be too narrow to result in a crossing of threshold voltage 110. But as the data rate is slowed, the user may over compensate for the slow response of the dimming switch such that the user begins to oscillate the dimming switch, resulting in user frustration.
Accordingly, there is a need in the art for improved signaling techniques through isolating devices such as an optocoupler.